Image sensors are semiconductor devices for converting optical images into electric signals.
Among the types of image sensors, a charge coupled device (CCD) includes MOS (metal oxide silicon) capacitors, which are aligned closely to each other to store or transfer charge carriers.
In contrast, a CMOS (complementary MOS) image sensor employs a switching mode to sequentially detect outputs of pixels by using MOS transistors prepared corresponding to the number of pixels through a CMOS technology that uses peripheral devices, such as a control circuit and a signal processing circuit.
Recently, the number of pixels of the CMOS image sensor has increased to a Mega level, so the size of the pixels has become reduced. Such a reduction in the size of the pixel causes restriction to the size of a micro-lens formed on the pixel, so that the focus length of the micro-lens has become shortened. For this reason, a metal interconnection layer constituting a logic circuit around the pixel is also limited.
As the number of pixels of the CMOS image sensor increases, improved interconnection processes are important in order to enlarge a pixel area.